Prediction of a two-dimensional high Curie temperature Weyl nodal line kagome semimetal.

Kagome lattices may have numerous exotic physical properties, such as stable ferromagnetism and topological states. Herein, combining the particle swarm structure search method with first-principles calculations, we identify a two-dimensional (2D) kagome Mo2Se3 crystal structure with space group P6/mmm. The results show that 2D kagome Mo2Se3 is a 100% spin-polarized topological nodal line semimetal and exhibits excellent ambient stability. The band crossing points form two nodal loops around the high-symmetry points Γ and K. On the other hand, Mo2Se3 shows intrinsic ferromagnetism with a large magnetic moment of 3.05 µB per Mo atom and magnetic anisotropy energy (MAE) of 4.78 meV. Monte Carlo simulations estimate that Mo2Se3 possesses a high Curie temperature of about 673 K. In addition, its ferromagnetic ground state can be well preserved under external strain, and the MAE can be improved by increasing the strain. More importantly, the position of each nodal line can be adjusted to the Fermi level through hole doping. This multifunctional 2D magnetic material that combines spin and topology has great potential in the field of nanoscale spintronic devices.

Theoretical Determination of the Electronic Structures and Energy-Level Splitting for Pr3+-Doped Y2SiO5 Crystals.

Trivalent praseodymium (Pr3+)-doped yttrium silicate (Y2SiO5) crystals have been widely used in various phosphors owing to their excellent luminescence characteristics. Although a series of studies have been carried out on its application prospects, the electronic structures and energy-transfer mechanisms of Pr3+-doped Y2SiO5 (Y2SiO5:Pr) remain an exploratory topic. Herein, the crystal structure analysis by the particle swarm optimization structure search method is used to study the structural evolution of Y2SiO5:Pr. Two novel structures with local [PrO7]-11 and [PrO6]-9 [Y2SiO5:Pr (I) and Y2SiO5:Pr (II)] are successfully identified. The impurity Pr3+ ions occupy the Y3+ sites and successfully integrate into the Y2SiO5 host crystal with a Pr3+ concentration of 6.25%. The calculated electronic band structures show that the doping of Pr3+ induces a reduction in band gaps for the host Y2SiO5 crystal. The conduction bands near the Fermi level are completely composed of f states. For the atomic energies of Pr3+ in Y2SiO5, the Stark levels and transitions are properly simulated based on a new set of crystal field parameters (CFPs) at the C1 site symmetry. A satisfactory r.m.s. dev. of 15.57 cm-1 with 9 free ion parameters (plus 27 fixed CFPs as obtained from ab initio calculation) fitted to the 33 observed levels is obtained for the first time. The plentiful energy-level transition lines, from the visible light to the near-infrared region, are deciphered for Pr3+ in Y2SiO5. Blue 3P0 → 3H4 at 465 nm is calculated to be a strong emission line, and it might be an ideal channel for laser actions. These results could not only provide important insights into the rare-earth-doped crystals but also lay the foundation for future research studies of designing the new laser materials.

Two-Dimensional GeC/MXY (M = Zr, Hf; X, Y = S, Se) Heterojunctions Used as Highly Efficient Overall Water-Splitting Photocatalysts.

Hydrogen generation by photocatalytic water-splitting holds great promise for addressing the serious global energy and environmental crises, and has recently received significant attention from researchers. In this work, a method of assembling GeC/MXY (M = Zr, Hf; X, Y = S, Se) heterojunctions (HJs) by combining GeC and MXY monolayers (MLs) to construct direct Z-scheme photocatalytic systems is proposed. Based on first-principles calculations, we found that all the GeC/MXY HJs are stable van der Waals (vdW) HJs with indirect bandgaps. These HJs possess small bandgaps and exhibit strong light-absorption ability across a wide range. Furthermore, the built-in electric field (BIEF) around the heterointerface can accelerate photoinduced carrier separation. More interestingly, the suitable band edges of GeC/MXY HJs ensure sufficient kinetic potential to spontaneously accomplish water redox reactions under light irradiation. Overall, the strong light-harvesting ability, wide light-absorption range, small bandgaps, large heterointerfacial BIEFs, suitable band alignments, and carrier migration paths render GeC/MXY HJs highly efficient photocatalysts for overall water decomposition.

Study of electronic and optical properties of CdI2 modulated by electric field: a first-principles study.

We found that an out-of-plane vertical electric field of 1.0 V/Ang helps to maintain the thermodynamic and kinetic stability of monolayer CdI2.The results indicated that the electric field modulates monolayer CdI2 to produce the Mexican-hat electronic state and the giant Stark effect of the vertical electric field on monolayer CdI2 originates from electric field lifting its conduction band. The results based on HSE06 + SOC calculations show that electric field induces strong spin polarization, leading to significant energy level splitting and spin flipping in the valence band. Based on GW0 + BSE, the electric field broadens effective optical response range of monolayer CdI2, the new peak in the optical absorption spectrum under electric field indicates that electric field helps to diminish excitonic effect of monolayer CdI2.

High thermoelectric performance of two-dimensional layered AB2Te4 (A = Sn, Pb; B = Sb, Bi) ternary compounds.

The thermoelectric properties of two-dimensional layered ternary compounds AB2Te4, in which A (Sn, Pb) and B(Sb, Bi) are group-IV and group-V cations, respectively, were investigated by using first-principles based transport theory. These septuple-atom-layer monolayers have wider band gaps with respect to their bulks, which extend their operating temperature and inhibit the bipolar carrier conduction and thermal conductivity, and more importantly, their energy bands exhibit multiple valence band convergence to a narrow energy range near the Brillouin zone center, which induces an optimal p-type power factor up to 10.94-32.11 W m-1 K-2 at room temperature. Moreover, these monolayers contain heavy atomic masses and high polarizability of some chemical bonds, leading to small group velocities of phonons and anharmonic phonon behavior that produce an intrinsic lattice thermal conductivity as low as 0.79-3.13 W m-1 K-1 at room temperature. Thus, these monolayers act as p-type thermoelectric materials with thermoelectric figure of merit of up to 2.6-5.5 for SnSb2Te4, 0.7-2.2 for PbSb2Te4, and 1.6-4.2 for PbBi2Te4 in the temperature range of 300 to 750 K, and 4.5-5.9 for SnBi2Te4 in the temperature range of 300 to 450 K.

Excellent 5f-electron magnet of actinide atom decorated gh-C3N4 monolayer.

Atomic functionality of two-dimensional (2D) materials, typically with a controllable doping route for offering regular atomic arrangement as well as excellent magnetism, is crucial for both fundamental studies and spintronic applications. Here, the adsorptions of the 5f-electron actinide series (An = Ac-Am) on porous graphene-like carbon-nitride (gh-C3N4) layers are explored to determine their structural stabilities, electronic nature and magnetic properties using the combination of density functional theory (DFT) calculations, ab initio molecular dynamics (AIMD), Monte Carlo (MC) simulations and chemical bonding analyses. Our investigations reveal that each An atom can be individually adsorbed at the vacancy site of gh-C3N4 sheet with high energetic, thermal and dynamical stabilities, which are rooted in the major interactions of ionic An-N bonding as well as the minor interactions of covalent bonding of An-5f6d states with N-2s2p states. The delocalization of a very few 5f electrons is dependent on whether they occupy the suborbitals that are matching and conducive to hybridize with the ligand orbitals forming the 5f-2s2p covalent bonds. We propose that the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism plays a determining role for the inter-atomic 5f-5f magnetic exchange via the 6d electrons as the conduction electrons. Large magnetic moment and magnetic anisotropy energy (MAE) from the localized 5f electrons, together with the metallic characteristics owing to the delocalized 6d electrons, render these An-based 2D materials excellent metallic magnets, especially for the U@gh-C3N4 system with the modest magnetic moment of 0.6 µB, large MAE of 53 meV and high Curie temperature (TC) of 538 K.

Thermoelectric response of single quintuple layer sodium copper chalcogenides persisting at high temperature.

The thermoelectric transport properties of two-dimensional (2D) layered NaCuX (X = S, Se) are investigated by employing first-principles based Boltzmann transport theory. Single quintuple NaCuX layers have a relatively large Seebeck coefficient (S), electrical conductivity (σ) and hence power factor (PF = S2σ) for a p-type heavy doped region due to the valence band degeneracy. The largely reduced σ by dominant polar scattering leads to a PF up to 0.27 and 0.84 mW m-1 K-2 at 1200 K for p-type NaCuS and NaCuSe monolayers, respectively. The high polarizability of the Cu-X bonds in the CuX4 tetrahedra leads to anharmonic phonon behavior which produces an intrinsic lattice thermal conductivity (κl) as low as 1.03 and 0.75 W m-1 K-1 at 300 K for NaCuS and NaCuSe, respectively. The predicted figure of merit (zT) increases monotonically from around 0.25 at 300 K to 2.01 at 1200 K at an optimal carrier density of around 1 × 1013 cm-2 for p-type NaCuSe and from around 0.09 at 300 K to 1.15 at 1200 K at an optimal carrier density of around 1 × 1014 cm-2 for p-type NaCuS. These findings indicate that the NaCuS, especially NaCuSe, monolayers are promising 2D thermoelectric materials persisting at high temperature.

Photoluminescence and energy transfer mechanisms of Tm3+ doped Y2O3 laser crystals: experimental and theoretical insights.

Rare-earth thulium (Tm3+) doped yttrium oxide (Y2O3) host single crystals are promising "eye-safe" laser materials. However, the mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals are not yet understood at the fundamental level. Here, we synthetize a series of Y2O3:Tm3+ samples by the sol-gel method. Our experimental results show that the most intensive absorption line of the 3H6 → 1D2 transition occurs at 358 nm, and the strongest emission line of the 1D2 → 3F4 transition is located at 453 nm, which are in good agreement with the calculations of 363 nm and 458 nm, respectively. By using the CALYPSO structural search method, the ground state structure of Y2O3:Tm3+ with P2 space group symmetry is uncovered. The complete energy levels, including free-ion LS terms and crystal-field LSJ multiplet manifolds, of Y2O3:Tm3+ are obtained based on our developed WEPMD method. The present findings show that our WEPMD method can be used in experiments to elucidate the underlying mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals, which offer insights for further understanding of other rare-earth doped laser materials.

Monolayer gadolinium halides, GdX2 (X = F, Cl, Br): intrinsic ferrovalley materials with spontaneous spin and valley polarizations.

Two-dimensional (2D) intrinsic ferrovalley semiconductors provide unprecedented opportunities to investigate valley physics as well as providing promising device applications due to their exceptional combination of spontaneous spin and valley polarizations. Here, we have predicted from first-principles calculations and Monte Carlo simulations that monolayers (MLs) GdX2 are such extremely rare excellent materials. Apart from their robust stabilities energetically, dynamically, thermally, and mechanically, these 2D materials are found to be semiconducting intrinsic ferromagnets where the magnetic coupling is ascribed to 5d-electron-mediated 4f-4f exchange interactions. Moreover, MLs GdX2 (X = F, Cl, Br) not only exhibit significant magnetic anisotropy energy of 351, 268, and 30 µeV per Gd, but also have a high Curie temperature of 300, 245, and 225 K, respectively. In particular, spontaneous valley polarization in three systems occurs due to the cooperative interplay between the spin-orbit coupling and magnetic exchange interactions, whose magnitude is as sizable as 55, 38, and 82 meV for MLs GdF2, GdCl2, and GdBr2, respectively. Under the action of an in-plane longitudinal electrical field, the valley-contrasting Berry curvatures arising from the broken space-inversion and time-reversal symmetries in MLs GdX2 could yield opposite transverse velocities of the carriers, giving rise to the occurrence of a spin-polarized anomalous valley Hall effect. Overall, these findings render 2D GdX2 a class of promising candidate materials for experimental studies and practical spintronics and valleytronics applications.

Hourglass Weyl and Dirac nodal line phonons, and drumhead-like and torus phonon surface states in orthorhombic-type KCuS.

In parallel to electronic systems, the concept of topology has been extended to phonons, which has led to the birth of topological phonons. In this Communication, based on symmetry analysis and first-principles calculations, we propose that hourglass Weyl nodal line (HWNL) phonons and Dirac nodal line (DNL) phonons coexist in the phonon dispersion of a single material, KCuS, with a Pnma-type structure. The HWNLs and DNLs are relatively flat in frequency and well separated from other phonon bands. The drumhead-like phonon surface state and the torus phonon surface state appear at the [001] and [100] surfaces, respectively. The reason for this phenomenon is explained based on the Zak phase calculations. The DNL and HWNL phonons are symmetry-related and these phonons can also be observed in other realistic materials, such as BaSi2, TiB and ZrSi, with a Pnma-type structure. Our Communication, for the first time, proves that phononic nodal lines with different types of degeneracies and different types of phonon surface states can be achieved in one single material. Thus, KCuS with a Pnma-type structure can be viewed as a good platform to investigate the entanglement between HWNL phonons and DNL phonons and to realize the drumhead-like and torus phonon surface states.

Above-room Curie temperature and barrier-layer-dependent tunneling magnetoresistance in 1T-CrO2 monolayer based magnetic tunnel junctions.

van der Waals (vdW) heterostructures based on two-dimensional (2D) ferromagnetic materials hold great potential applications in spintronics. Using the density functional theory (DFT) method and first-principles quantum transport simulation, we studied the structures, magnetic properties and spin-resolved transport of 1T-CrO2 monolayer (ML) based vdW magnetic tunnel junctions (MTJs). Owing to a high Curie temperature (TC) of 392 K and a moderate magnetic anisotropy energy (MAE) of 94 µeV of the ferromagnetic 1T-CrO2 monolayer, Cu(111)|CrO2|nML-Gr|CrO2|Cu(111) MTJs were built. Our results reveal that their tunneling magnetoresistance (TMR) ratios are dependent on the number of Gr barrier layers within a working bias voltage of 1 V. For the thin barrier layers (n = 1-2), the maintained TMR ratios can reach a giant value of about 1 × 104%, while there appears a decreasing trend with the increasing bias voltage for thick Gr layers (n = 3-5). The barrier-layer-dependent phenomenon is attributed to the decreasing transmission magnitude with increasing bias voltage in a parallel configuration (PC), which is as small as that in an anti-parallel configuration (APC) eventually. Our results would provide some guidance for future experimental fabrications of these 2D materials based MTJs.

Two-dimensional CdS/SnS2 heterostructure: a highly efficient direct Z-scheme water splitting photocatalyst.

A desired water splitting photocatalyst should not only possess a suitable bandgap and band edge position, but also host the spontaneous progress for overall water splitting without the aid of any sacrificial agents. In this work, we propose a two-dimensional CdS/SnS2 heterostructure (CSHS) as a possible water splitting photocatalyst by first-principles calculations. The CSHS enhances the absorption of visible and infrared light, and the type-II band alignment guarantees the spatial separation of the photoinduced carriers. The induced built-in electric field across the CSHS interface efficiently separates the photoexcited carriers and extends their carrier lifetimes. All these properties make the CSHS a direct Z-scheme system with the hydrogen and oxygen evolution reactions occurring, respectively, at the CdS and SnS2 layers. More encouragingly, the introduction of a S-vacancy into SnS2 could effectively lower the overpotential of the oxygen evolution reaction, thus ensuring the overall water redox reaction to be achieved spontaneously under light irradiation. Our findings suggest that the CSHS is a promising water splitting photocatalyst.

A two-dimensional PtS2/BN heterostructure as an S-scheme photocatalyst with enhanced activity for overall water splitting.

Photocatalytic hydrogen production from water is a sustainable solution to the environmental pollution and energy crises. Encouraged by the successful synthesis of PtS2 and BN nanosheets and their suitable band edges, we have designed a PtS2/BN bilayer heterojunction and investigated its electronic and optical properties for the first time based on hybrid DFT calculations. In this system, the built-in electric field and band edge bending can retain useful electrons on the conduction band of BN and holes on the valence band of PtS2, which endow this system with a stronger redox ability. Meanwhile, this electric field can efficiently separate photoinduced electron-hole pairs and improve the photocatalytic efficiency. Compared with BN and PtS2 single layers, the PtS2/BN heterojunction with its smaller bandgap can make better use of visible and infrared light. Additionally, we have studied the effect of applied strain on the electronic and optical properties. This work aims to provide a method for constructing high-efficiency BN-based photocatalysts and illuminating the electron migration mechanism in step-scheme (S-scheme) heterostructures. We have found that the PtS2/BN bilayer heterojunction is a promising S-scheme photocatalyst for overall water decomposition.

Insights into the Microstructures and Energy Levels of Pr3+-Doped YAlO3 Scintillating Crystals.

Trivalent praseodymium (Pr3+)-doped materials have been extensively used in high-resolution laser spectroscopy, owing to their outstanding conversion efficiencies of plentiful transitions in the visible laser region. However, to clarify the microstructure and energy transfer mechanism of Pr3+-doped host crystals is a challenging topic. In this work, the stable structures of Pr3+-doped yttrium orthoaluminate (YAlO3) have been widely searched based on the CALYPSO method. A novel monoclinic structure with the Pm group symmetry is successfully identified. The Pr3+ impurity can precisely occupy the Y3+ position and get incorporated into the YAlO3 (YAP) host crystal with a Pr3+ concentration of 6.25%. The result of the electronic band structure reveals a 3.62 eV band gap, which suggests a semiconductor character of YAP:Pr. Using our developed well-established parametrization matrix diagonalization (WEPMD) method, we have systematically analyzed the energy level scheme and proposed a set of newly improved parameters. Additionally, the energy transfer mechanism of YAP:Pr is clarified by deciphering the numerical electric dipole and magnetic dipole transitions. The popular red emission at 653 nm is assigned to the transition 3P0 → 3F2, while the transition 3P0 → 3H4 with a large branching ratio is predicted to be a good laser channel. Many promising emission lines for laser actions are also obtained in the visible light region. Our results not only provide important insights into the energy transfer mechanisms of rare-earth ion-doped materials but also pave the way for the implementation of new types of laser devices.

Study of photogenerated exciton dissociation in transition metal dichalcogenide van der Waals heterojunction A2-MWS4: a first-principles study.

In order to explore the photocatalytic hydrogen production efficiency of the MoS2/WSe2 heterostructure (A2-MWS4) as a photocatalyst, it is highly desirable to study the photogenerated exciton dissociation related to photocatalysis. The electronic properties, optical absorption, and lattice dynamic properties of A2-MWS4 were investigated using a first-principles approach. The results show that the type II energy band alignment of A2-MWS4 facilitates the dissociation of photogenerated excitons (electrons and holes). The highly localized d-state electrons of A2-MWS4 induce the formation of internal potentials that promote the dissociation of photogenerated excitons. The hot carrier diffuses its extra energy into the lattice by scattering with phonons and forms a hot spot in the lattice while releasing phonons, which are dragged away from the hot spot by Ridley decay to promote exciton dissociation. These findings could provide insights for research studies on photochemical reactions and photovoltaic devices.

Two dimensional CdS/ZnO type-II heterostructure used for photocatalytic water-splitting.

The electronic structures of two dimensional (2D) CdS/ZnO heterostructure (CdZnHT) consisting of CdS singlelayer (SL) and ZnO SL are explored based on hybrid density functional calculation. The negative interface formation energies suggest the formation of CdZnHT is exothermic. The bandgap of CdZnHT is favorable for absorbing visible light, and the decent band edge position makes it thermodynamically feasible for spontaneous generation of oxygen and hydrogen. The formed electric field across the interface induced by charge transfer will reduce photogenerated carrier recombination and promote carrier migration. Particularly, CdZnHT is a type-II heterostructure. Oxygen generation takes place at ZnO layer and hydrogen production occurs at CdS layer, which will also promote the effective separation and migration of phogogenerated carriers and enhance photocatalytic performance. These findings suggest that 2D CdZnHTs are possible candidates as water-splitting photocatalysts.

Band gap and magnetic engineering of penta-graphene via adsorption of small transition clusters.

Penta-graphene has been intensively studied owing to its superior properties such as being an intrinsic semiconductor and having two dimensional stability. However, the nonmagnetic character makes it difficult for straightforward application in the fields of spintronic or information storage. Here, the deposition effects of Fe-group and Co-group transition metal (TM = Fe, Ru, Os; Co, Rh, Ir) clusters on the penta-graphene have been systemically investigated for their electronic and magnetic properties by using density functional theory (DFT) calculations. We found that the TM deposition stability on penta-graphene is overall greater than that on graphene. Importantly, TM adatoms (adclusters) not only change penta-graphene from being a wide band-gap semiconductor to a narrow band-gap semiconductor, but also introduce large magnetic moments into systems simultaneously. It is worth noting that the Ir5 cluster on penta-graphene is a good candidate for realizing the magnetic half-metallic materials. Our calculated results demonstrate that adatoms can exhibit large out-of-plane magnetic anisotropy energy, e.g., the Os adatom presents the largest value of 113 meV. Therefore, from the application point of view, magnetic functionalization of penta-graphene by TM clusters facilitates its application as a spintronic device or a high-density information storage device.

A two-dimensional h-BN/C2N heterostructure as a promising metal-free photocatalyst for overall water-splitting.

The construction of a heterostructure (HS) is an effective strategy to modulate the desired properties of two-dimensional (2D) materials and to extend their applications. In this paper, based on the density functional theory, we predict a metal-free type-II HS formed by h-BN and C2N single layers. The h-BN/C2N HS possesses a smaller bandgap than individual h-BN and C2N single layers, and it exhibits excellent visible light absorption. Importantly, its band edge positions satisfy the requirements for spontaneous water-splitting. With the assistance of the built-in electric field across the HS and the band offset, the photoinduced carriers can be readily spatially separated. Free energy calculations indicate the high catalytic activity for water oxidation and reduction reactions. The performance can be further enhanced by strain, which modulates the bandgap and the band edge positions of the HS. The band alignment may undergo a transition from type-I to type-II under strain, offering an effective switch for the reaction. The appropriate bandgap, suitable band edge positions, and effective carrier separation make the h-BN/C2N HS a promising candidate for use as a photocatalyst in water-splitting.

Robust spin manipulation in 2D organometallic Kagome lattices: a first-principles study.

The search for 2D ferromagnets with versatile magneto-electronic properties is becoming more active due to their potential applications in spintronic devices. To screen out the optimal compositions, we have explored a series of two-dimensional M3C12X12 (M = 5d transition metals, and X = S, NH, and O) metal-organic frameworks with Kagome lattice patterns through first-principles calculations. By varying the metal center and ligand functional radicals, both the electronic and spin-related properties can be easily tuned to meet the requirements for multifunctional applications in spintronic devices. Among them, Re3C12N12H12 is identified to be a ferromagnetic bipolar magnetic semiconductor with the highest Curie temperature (TC > 330 K). Re3C12O12 is found to be an ideal half-metal with a spin gap of 0.97 eV, which is beneficial for use as a spin-filter. Meanwhile, both Re3C12N12H12 and Re3C12O12 exhibit considerable out-of-plane magnetic anisotropy energies (>26 meV per atom), which benefit the spintronic applications. The theoretical results not only show that the 2D organometallic Kagome lattice is a good platform for designing spintronic materials, but also provides a feasible way to realize robust spin manipulation.

Enhanced Stability and Optical Absorption in the Perovskite-Based Compounds MA1-x Csx PbI3-y Bry.

Organometal halide perovskites have been outstanding from enormous amount of functional materials thanks to their highly cost-effective processability and prominent light harvesting capacity. Unfortunately, poor long-term stability seriously hinders their further development. The recent experimental observations suggest that Cesium is a promising candidate to enhance the stability of MAPbI3 . To explore the inherent mechanism, a first-principles investigation based on density functional theory, including hybrid functional, has been performed to analyze the electronic and optical properties of perovskite series MA0.75 Cs0.25 PbI3-y Bry . The results indicate that perovskite compound MA0.75 Cs0.25 PbI2 Br is significantly superior to the other doped series in terms of optical absorption within the visible-light range. In the meanwhile, both Bader charge analysis and charge density distribution show that the compound of MA0.75 Cs0.25 PbI2 Br is the most stable among all the doped perovskite series. Moreover, it is clearly manifested that the impact of cesium is mainly embodied in the enhancement of the stability rather than in the improvement of optical absorption. Our study sheds a new light on screening new-type light harvesting materials, and provides theoretical insight into the rationale design of highly efficient and stable photovoltaic devices based on these functional materials.